1. Field of the Invention
The present invention relates to an optical modulator and the like. In particular, the present invention relates to a DSB-SC modulation system capable of performing high extinction ratio modulation by suppressing a carrier component and a high order component, such as a second order component generated, when DSB modulation is performed.
2. Description of the Related Art
In optical communication, light is modulated to have information. As optical modulation, a direct modulation and an external modulation are known. The direct modulation modulates a driving power of semiconductor laser. The external modulation modulates light from semiconductor laser by means of other than light source. A modulator used in direct modulation is generally called as an optical modulator. The optical modulator modulates optical intensity, phase, etc. by causing physical changes in the optical modulator based on applied signals. The optical modulator has technical problems that it is difficult to attain reduction of driving voltage, realization of a higher extinction ratio for improving modulation efficiency, widening a bandwidth, and improvement of high light utilization efficiency for speeding up and loss reduction of a modulation. In other words, development of a modulator having high extinction ratio is desired. It is to be noted that the extinction ratio is a ratio of optical intensity of the highest level to the lowest level.
An optical signal side-band (optical SSB) modulator is a modulator which shifts frequency of an optical signal and outputs the optical signal [Tetsuya Kawanishi and Masayuki Izutsu, “Optical frequency shifter using optical SSB modulator”, TECHNICAL REPORT OF IFICE, OCS2002-49, PS2002-33, OFT2002-30 (2002-08)].
An optical FSK modulator which is a modification of an optical SSB modulator is also known [Tetsuya Kawanishi and Masayuki Izutsu, “Optical FSK modulator using an integrated light wave circuit consisting of four optical phase modulator”, CPT 200407-2, Tokyo, Japan, 14-16 Jan. 2004][Tetsuya Kawanishi, et al. “Analysis and application of FSK/IM simultaneous modulation” Tech. Rep. of IEICE. EMD 2004-47, CPM 2004-73, OPE 2004-130, LQE 2004-45 (2004-08), pp. 41-46].
FIG. 9 is a schematic diagram showing a basic arrangement of a conventional optical modulation system acting as an optical SSB modulator or an optical FSK modulator. As shown in FIG. 9, this optical modulation system comprises a first sub Mach-Zehnder waveguide (MZA) (2), a second sub Mach-Zehnder waveguide (MZB) (3), a main Mach-Zehnder waveguide (MZC) (8), a first electrode (RFA electrode) (9), a second electrode (RFB electrode) (10), and a modulation electrode.
The main Mach-Zehnder waveguide (MZC) (8) includes an input part (4) of an optical signal, a branching part (5) where the optical signal is branched to the first sub Mach-Zehnder waveguide (MZA) and the second sub Mach-Zehnder waveguide (MZB), the first sub Mach-Zehnder waveguide (MZA), the second sub Mach-Zehnder waveguide (MZB), a multiplexing part (6) multiplexing the optical signal outputted from the first sub Mach-Zehnder waveguide (MZA) and the second sub Mach-Zehnder waveguide (MZB), an output part (7) outputting the optical signal multiplexed by the multiplexing part.
The first electrode (RFA electrode) (9) inputs radio frequency (RF) signals to two arms comprising the first sub Mach-Zehnder waveguide (MZA).
The second electrode (RFB electrode) (10) inputs radio frequency (RF) signals to two arms comprising the second sub Mach-Zehnder waveguide (MZB).
The modulation electrode is provided on the main Mach-Zehnder waveguide.
Changing USB and LSB, which can be used as information, are attained by means of electrode of the main Mach-Zehnder waveguide; thereby frequency shift keying is realized.
As an optical modulator, an optical double side-band suppressed carrier (DSB-SC) modulator is publicly known. The above described optical modulation system also acts as a DSB-SC modulator. The DSB-SC modulator ideally outputs two side bands, suppressing carrier components. However, in reality, in an output of a DSB-SC modulator shown in the figure below, unsuppressed carrier component (f0) and a high order component (e.g. a second order component (f0±2fm)) remain, preventing extinction ratio from improving.
For example, a DSB-SC modulator having a Mach-Zehnder, PMs provided on both arms of the Mach-Zehnder and a fixed phase shifter provided on one arm of the Mach-Zehnder is disclosed in FIG. 37 of Japanese Unexamined Patent Application Publication No. 2004-252386. An optical DSB-SC modulator ideally outputs two sideband (double sideband) signals, thereby suppressing carrier signal components. However, odd order components remain slightly. As such a remaining component, a third order component is theoretically considered to be the strongest.
It is an object of the present invention to provide a new optical modulation system.
It is an object of the present invention to provide a DSB-SC modulation system which is capable of suppressing a carrier component (f0) and a high order component (such as a second order component (f0±2fm)).